Energy recovery in a wellbore

ABSTRACT

A process for the recovery of energy from a pressurised well stream containing a gas/liquid mixture, the process comprising: treating the well stream to a pre-separation process to separate it into gaseous and liquid phases, selecting appropriate proportions of said separated gaseous and liquid phases, recombining said selected proportions, and supplying the recombined mixture to the inlet of a rotary separation turbine, wherein said components are separated and energy is recovered from the flow by rotation of the turbine, said proportions of said gaseous and liquid phases being selected to produce an optimum mixture for supply to the rotary separation turbine.

This invention relates to a process and apparatus for the recovery ofenergy from a gas/liquid mixture, primarily a gas and oil/water mixturefrom an oil well.

The general concept of recovering energy from a well stream, whether itbe a hydrocarbon well, or a geothermal well, is known. For example, U.S.Pat. No. 5,385,446 shows the use of a rotary separation turbine torecover energy, from, and separate the constituents of a gas liquidmixture from a geothermal well. U.S. Pat. No. 5,117,908 shows that it isknown to use a rotary turbine to recover energy from the gas/liquidmixture in the well stream of an oil well as a stage prior to theseparation of the gas/liquid mixture.

Rotary separation turbines, for example of the kind illustrated in U.S.Pat. No. 5,385,446 incorporate a specifically designed nozzle throughwhich the inlet mixture is directed into the rotary separation turbine.The present invention is based upon the recognition that such rotaryseparation turbines are designed to operate with optimum efficiency whensupplied at a predetermined flow rate, with a mixture having apredetermined gas/liquid ratio.

In accordance with the present invention there is provided a process forthe recovery of energy from a pressurised well stream containing agas/liquid mixture, the process comprising treating the well stream to apre-separation process to separate it into gaseous and liquid phases,selecting appropriate proportions of said separated gaseous and liquidphases, recombining said selected proportions, and supplying therecombined mixture to the inlet of a rotary separation turbine whereinsaid components are separated and energy is recovered from the flow byrotation of the turbine, said proportions of said gaseous and liquidphases being selected to produce an optimum mixture for supply to therotary separation turbine.

The invention further resides in an apparatus for recovering energy froma well stream comprising a pre-separation device for separation of thewell stream into gaseous and liquid components, selection means forselecting predetermined proportions of said gaseous and liquidcomponents, mixing means for recombining said selected proportions, anda rotary separation turbine driven by said recombined mixture.

One example of the invention is illustrated in the accompanying drawingswherein

FIG. 1 is a diagrammatic representation of a basic energy recoveryprocess and apparatus;

FIG. 2 is a diagrammatic representation of an enhancement of the processand apparatus illustrated in FIG. 1;

FIG. 3 is a diagrammatic representation of a modification of thearrangement illustrated in FIG. 2 in which more than one rotaryseparation turbine may be supplied from more than one well stream, and,

FIG. 4 is a diagrammatic representation of a further modification.

Referring first to FIG. 1 of the drawings, the well stream 11 of an oilwell or other hydrocarbon well (or a proportion thereof) containing agas/liquid mixture, usually, gas, oil, and water is supplied to theinlet of a gas/liquid cyclone separator 12 which separates the wellstream 11 into its gaseous and liquid phases without any significantpressure loss. The gaseous phase of the well stream issues from thecyclone separator by way of its reject outlet 13 while the liquid phaseissues from the underflow outlet 14 of the separator 12.

The reject outlet 13 is connected to a mixing device 15 through a line16 containing a control valve 17. In addition, the reject outlet 13 isconnected to a gas discharge line 18 through a control valve 19. A line21 connects the underflow outlet 14 with the mixer 15, the line 21including a control valve,22 and in addition the underflow outlet 14 isconnected to a liquid discharge line 23 through a control valve 24. Themixer 15 has an outlet line 25 coupled to the inlet nozzle of a rotaryseparation turbine 26 which has gas and liquid outlet ports 27, 28connected respectively to the gas and liquid output lines 18, 23.

Taking a simplistic, overview of the process and apparatus of FIG. 1,the control valves 17, 19 and 22, 24 are set by an operator to achievethe supply to the mixer 15, at predetermined flow rates and pressure, ofa predetermined ratio of the gaseous and liquid phases issuing from theseparator 12.

The rotary separation turbine 26, and in particular its inlet nozzle,will have been designed to operate most efficiently when supplied, at apredetermined flow rate and pressure, with a mixture containing apredetermined gas/liquid ratio. The valves 17, 19, 22, 24 are thusadjusted to ensure that appropriate proportions of the gaseous andliquid phases issuing from the separator 12 are routed to the mixer 15where they are recombined for supply to the inlet nozzle of the rotaryseparation turbine 26.

Within the rotary separation turbine, the recombined gaseous phaseflashes out of the gas/liquid mixture as the mixture passes through theinlet nozzle of the turbine thus accelerating the liquid phase onto therotary component of the turbine and driving the rotary component.Rotational energy of the rotating component of the turbine (and thus ofthe well stream) can be recovered in a number of ways, for example bycoupling an electrical generator to the shaft of the rotary component,or by using scoops dipping into a liquid layer on the rotating componentto derive a pressurised liquid supply from the rotary separator. Themanner in which the energy is “tapped” from the rotary separationturbine is not of importance to the present invention, and will bedetermined, to a large extent, by the nature of the turbine which hasbeen selected.

It will be recognised that in addition to recovering energy from thewell stream the rotary separation turbine separates the recombinedportion of the well stream into at least its gaseous and liquidcomponents for furth, processing. Where the liquid component containsoil and water then the rotary separation turbine 26 can be designed toeffect separation of the liquid phase into its different densitycomponents.

The arrangement described with reference to FIG. 1 cannot respond tochanges in the composition of the well stream. The apparatus illustratedin FIG. 2 is an enhancement of the arrangement illustrated in FIG. 1,and depicts a practical application of the principles disclosed in FIG.1 in which changes in well stream composition can be accommodatedautomatically.

It can be seen that the cyclone separator 12 is housed within a pressurevessel 31, the inlet for the separator 12 being ducted through the wallof the vessel 31. The separator 12 discharges the gaseous and liquidcomponents separated from the well stream 11 into the vessel 31, suchthat the upper part of the vessel 31 is filled with gas while the lowerpart is filled with liquid, the liquid level being illustrated in FIG. 2at 32. The upper wall of the vessel 31 has a gas outlet 13 a connectedthrough the line 16 to one inlet of the mixer 15, the valve 17 beingdisposed in the line 16 as described above.

The lower wall of the vessel 31 has a liquid outlet 14 a connectedthrough the line 21 and the valve 22 to the mixer 15. As described abovethe outlet 13 a is connected to the gas discharge line 18 through valve19 and the outlet 14 a is connected through valve 24 to the liquidoutput line 23. However, the valves 19 and 24 are arranged to be capableof automatic operation. The valve 19 is controlled automatically by apressure sensor arrangement 33 monitoring the pressure in the gas line16 adjacent the outlet 13 a. The valve 24 is controlled by a liquidlevel sensor arrangement 34 which monitors the liquid level 32 withinthe vessel 31 and supplies a control signal to the valve 24. It will beunderstood that the exact manner in which signals derived in relation togas pressure and liquid level are utilised to operate the valves 19 and24 is not of importance to the invention.

The setting of the valves 17, 22 determines the proportions of gas andliquid supplied to the mixer 15 and thus the gas/liquid ratio of themixture supplied at controlled pressure and flow to the inlet nozzle ofthe turbine 26. The valves 19, 24 are controlled to bypass excess gasand liquid respectively from the lines 16, 21 so as to maintainpredetermined pressure and flow characteristics in the lines 16, 21dictated by the settings of the valves 17, 22. Provided that thepressure and makeup of the well stream 11 remain within a predeterminedrange then the control regime compensates automatically for variationsin the parameters of the well stream 11 to maintain the supply to theline 25 optimised in relation to the chosen rotary turbine separator 26.

In many applications the valves 17, 22 will be manually operable devicesadjusted during a set-up phase to give the desired gas/liquid ratio atthe mixer 15. However, it is to be understood that if desired automatedcontrol of the valves 17, 22 is possible.

FIG. 2 illustrates that the well stream 11 may be derived from aplurality of wells rather than just a single well, the individual wellstreams being fed into a single manifold or supply line where they mixprior to being passed to the inlet of the cyclone separator 12. Clearlyadding or removing one or more streams to or from the combined wellstream can generate significant variations in the well streamparameters, which ordinarily would render the mixture fed to the turbinesome way from optimum. The system described above with reference to FIG.2 can accommodate such variations, maintaining the optimum mixturesupply to the turbine 26.

FIG. 2 illustrates a gravity separator 36 of conventional form,downstream of the turbine 26. The gravity separation vessel has a liquidinlet receiving liquid from the discharge line 23, and the outlet 28 ofthe turbine. In addition the gravity separation vessel has a gas inletreceiving the separated gas from the outlet 27 of the turbine. The gasdischarge line 18 from the cyclone separator 12 is shown, forconvenience, communicating with the liquid discharge line 23 adjacentthe vessel 36. It is to be understood however that if desired the gasdischarge line 18 could communicate with the gas discharge from theturbine 26, provided that the pressures are appropriately matched. Theturbine 26 recovers energy from the well stream as described above, andthe gravity separator 36 completes the separation of the well streaminto gaseous and liquid phases. Moreover, where the liquid phase is amixture of oil and water the gravity separator can, if desired, bearranged to permit gravity separation of the oil and water, although asdrawn in FIG. 2 the separator 36 has only a gas outlet and a liquidoutlet. Where three phase separation occurs in the separator 36 therewill be gas, oil and water outlets. It is to be recognised however thatit is not essential that the final stage of separation is a gravityseparator, and other known separation techniques can be used at thispoint, including the use of further cyclone separators and/or furtherturbine separators.

FIG. 3 illustrates a process and apparatus similar to that describedabove with reference to FIG. 2, but utilising a plurality of cycloneseparators performing the pre-separation of the well stream or wellstreams. It will of course be understood that in a variant of FIG. 2 aplurality of cyclone separators each having its own pressure vessel andeach having its own associated pressure and liquid level sensors couldbe utilised. However, FIG. 3 illustrates a refinement of such a multiplecyclone arrangement in which each cyclone has its own respective liquidlevel control system, but all of the cyclones share a common gaspressure control system. Thus referring specifically to FIG. 3 it can beseen that the first and second gas/liquid cyclone separators 12, 112receive respective well streams 11, 111, although in practice the wellstreams 11, 111 may be parts of a common well stream derived from one ormore wells, or may be separate well streams from respective wells. Eachcyclone separator 12, 112 is housed within a respective pressure vessel31, 131 having respective gas and liquid outlets 13 a, 14 a and 113 a,114 a as described above. A respective liquid level monitoringarrangement 34, 134 monitors the liquid level within the respectivepressure vessel and controls a respective valve 24, 124 determining howmuch of the liquid phase separated by the respective cyclone separatorbypasses the mixing arrangement and flows to a common liquid dischargeline 23. The predetermined remainder of the liquid output from each ofthe cyclone separators flows through a respective line 21, 121 into acommon liquid manifold 51.

The gas outlets 13 a, 113 a of the vessels 31, 131 are connected throughrespective lines 16, 116 to a common gas line 52 supplying a gasmanifold 53. A gas pressure monitoring arrangement 33 monitors the gaspressure in the line 52 and supplies a control signal to a valve 19 tocontrol the amount of gas which bypasses the mixing arrangement andflows to a common gas discharge line 18. It will be recognised that asdescribed with reference to FIG. 2 the valves 17, 22 (117, 122; 217,222; 317, 322) adjacent each mixer set the gas/liquid ratio for theirrespective mixer. The valve 19 is controlled to bypass gas which isexcess to the “demand” of the mixers to the output line 18, and thus thecontrol of the valve 19 ensures that the pressure stays within itsoperating limits and provides stable flow characteristics of lines 51and 53. Similarly the control of valves 24 and 124 ensures that excessliquid bypasses the mixers to the output line 23.

The arrangement illustrated in FIG. 3 is intended to supply fourseparate, substantially identical rotary separation turbines (notshown). Thus in relation to each of the turbines there is provided arespective mixer 15, 115, 215, 315 supplied with gas and liquid from themanifolds 53, 51 through respective valves equivalent to the valves 17,22 of FIG. 2. Each mixer has a respective output line connected to thenozzle of its respective turbine. It will be recognised that thesettings of the valves in the lines connecting each manifold 51, 53 tothe respective mixer control determine the gas/liquid ratio of themixture supplied to the respective turbine inlet nozzle, and each valvecan be finely adjusted to accommodate minor differences in specificationbetween the otherwise identical rotary separation turbines. Furthermore,while FIG. 3 illustrates only first and second cyclone separators, itwill be understood that exactly the same principle can be applied with agreater number of cyclone separators. Similarly, although FIG. 3illustrates the supply to four rotary separation turbines it is to beunderstood that more, or fewer, turbines can be accommodated if desired.

FIG. 4 illustrates a modification which may be used with any of thearrangements illustrated in FIGS. 1 to 3 where the rotary separationturbine 26 has a plurality of separate inlet nozzles. FIG. 4 disclosesan arrangement in which the rotary separation turbine has four angularlyspaced inlet nozzles, together with a gas outlet 27 and a liquid outlet28.

The appropriate proportions of gas and liquid, conveniently derived inthe manner described with reference to any one of FIGS. 1, 2 and. 3above is supplied through lines 116 and 121 respectively to gas andliquid manifolds 153, 151 of the rotary separation turbine. Line 116includes a control valve 117 for setting the gas proportion of thesupply to the manifolds while line 121 includes a similar valve 122 forsetting the liquid proportion of the supply to the manifolds. Themanifolds 151 and 153 encircle the fixed housing of the rotaryseparation turbine, and each is connected to a respective gas/liquidmixer 64, 164, 264, 364 which supplies a respective turbine inlet nozzlethrough a respective line 65, 165, 265, 365. Thus each mixer recombinesthe appropriate proportions of gas and liquid for supply to the inletnozzles of the rotary separation turbine at a point immediately adjacentthe nozzle.

The arrangement shown in FIG. 4 overcomes the difficulty of dividing amixed flow into four separate parts to supply the four nozzlesrespectively. Mixed (multiphase) flows are difficult to divideaccurately, and the FIG. 4 arrangement obviates the problem by dividingthe liquid phase into four parts, one for each nozzle; dividing the gasphase into four parts, again one for each nozzle; and then recombiningthe gas and liquid parts individually in a mixer specific to, andclosely adjacent a respective nozzle.

While the use of one or more gas/liquid cyclone separators as thepre-separation stage of the above described apparatus and processes ispreferred, it is to be recognised that other separator devices withassociated sensors could be utilised as the pre-separation stage.

What is claimed is:
 1. A process for the recovery of energy from apressurized well stream from a hydrocyclone well, the stream containinga gas/liquid mixture, the process comprising treating the well stream toa pre-separation process to separate it into gaseous and liquid phases,selecting appropriate proportions of said separated gaseous and liquidphases, recombining said selected proportions, and supplying therecombined mixture to the inlet of a rotary separation turbine whereinsaid components are separated and energy is recovered from the flow byrotation of the turbine, said proportions of said gaseous and liquidphases being selected to produce an optimum mixture for supply to therotary separation turbine.
 2. A process as claimed in claim 1, whereinthe pre-separation process is performed in a cyclone separator.
 3. Aprocess as claimed in claim 1, wherein the pre-separation and selectionprocess is responsive to changes in the composition of the well stream.4. A process as claimed in claim 1, wherein the well stream is acombined stream derived from a plurality of hydrocarbon wells by mixingthe individual streams from the wells.
 5. A process as claimed in claim1, wherein the liquid component of the well stream is a mixture ofliquids of different densities and the rotary separation turbine isarranged to separate the liquid component into at least two constituentparts.
 6. A process as claimed in claim 1, wherein there is a pluralityof cyclone separators in the pre-separation process.
 7. A process asclaimed in claim 1, wherein recombined mixture is supplied to aplurality of rotary separation turbines.
 8. An apparatus for recoveringenergy from a well stream comprising a pre-separation device forseparation of the well stream into gaseous and liquid components,selection means for selecting predetermined proportions of said gaseousand liquid components, mixing means for recombining said selectedproportions, and a rotary separation turbine driven by said recombinedmixture.
 9. Apparatus as claimed in claim 8, wherein the pre-separationdevice is a cyclone separator.
 10. Apparatus as claimed in claim 8,wherein the pre-separation device includes a plurality of cycloneseparators.
 11. Apparatus as claimed in claim 8, wherein the selectionmeans is controlled by monitoring means monitoring the composition ofthe well stream whereby said selection means is responsive to variationin the composition of the well stream in use.
 12. Apparatus as claimedin claim 11, wherein the pre-separation device is housed within apressure vessel which receives the well stream components separated bythe pre-separation device, and valves for discharging gas and liquidfrom the vessel in accordance with the liquid level and gas pressurewithin the vessel so that gaseous and liquid components of the wellstream, in appropriate proportions, can be recombined for supply to therotary separation turbine.
 13. Apparatus as claimed in claim 8,including manifold means upstream of the pre-separation device formixing individual streams from a plurality of wells to produce the wellstream supplied to the pre-separation device.
 14. Apparatus as claimedin claim 8, wherein the rotary separation turbine discharges theseparated gas and liquid components of the well stream into a gravityseparator in which further separation of gaseous and liquid componentstakes place.
 15. Apparatus as claimed in claim 8, wherein the recombinedgaseous and liquid components from the selection means is supplied to aplurality of rotary separation turbines.
 16. Apparatus as claimed inclaim 8, wherein the pre-separation device includes a plurality ofcyclone separators, each cyclone separator being housed within its ownpressure vessel, each pressure vessel having its own liquid levelcontrol system, but there being a common gas pressure control systemserving all of the vessels, the gas pressure control system and theindividual liquid level control systems each producing control signalscontrolling the proportions of gas and liquid supplied from each vesselfor recombination and onward supply to the rotary separation turbine orturbines.
 17. Apparatus as claimed in claim 8, in which the means formixing, for recombining said selected proportions of said gaseous andliquid components is positioned in close proximity to the inlet nozzleof the rotary separation turbine.
 18. Apparatus as claimed in claim 17,wherein the rotary separation turbine has a plurality of inlet nozzles,each inlet nozzle has a mixing means positioned closely adjacentthereto, and each mixing means is supplied with gaseous and liquidcomponents of the well stream, for recombination, by way of respectivegas and liquid component manifolds.
 19. Apparatus as claimed in claim18, in which each manifold encircle the fixed housing of the rotaryseparation turbine.